The present disclosure relates to an energy storage device, and particularly to an energy storage device including at least one nanostructure electrode having a large surface area of a pseudocapacitive material for pseudocapacitive energy storage, and methods of manufacturing the same.
Ultracapacitors or electrochemical double layer capacitors (EDLC's) provide the highest energy density among commercially available devices employing capacitive energy storage. Although such EDLC's are capable of operation at considerably higher power than a battery, the energy density of even high performance EDLC's is lower than the energy density of high performance batteries by a factor of 10˜20. A traditional ultracapacitor consists of two electrodes that are fabricated from highly porous activated carbon sheets that provide very large surface area, which is typically on the order of 1000 square meters/gram of material. These porous activated carbon-based electrodes are immersed in an electrolyte. When a voltage is applied across a porous activated carbon-based electrode and the electrolyte, energy is stored in the electric field set up in the double layer formed between the carbon surface and the electrolyte. No charge is transferred across the interface between the porous activated carbon-based electrode and the electrolyte.
The capacitance of an EDLC is thus limited by the area of the surface of the activated carbon sheets. Increasing this area is not only difficult, but also produces only minimal increases in stored energy. To date, this constraint has limited the energy density of an ultracapacitor to below 10 Wh/kg. This value has not changed appreciably in more than 10 years.
Another means of increasing the energy density is to store charge through redox (reduction/oxidation) chemistries at the surface of certain metals and metal oxides. This Faradaic process involves the actual transfer of electrical charges between the surface of the metal oxide and the electrolyte. The change in the stored electrical charges varies continuously as a function of an externally applied voltage in a manner similar to a conventional capacitor. Thus, this phenomenon is called pseudocapacitance. Pseudocapacitive energy storage refers to the method of energy storage employing the phenomenon of pseudocapacitance.
While pseudocapacitance (PC) can store about ten times more charge than a standard EDLC in theory, there are no commercial pseudocapacitors that have demonstrated anything remotely approaching this energy density level to this date. The problem can be found in the microscopic nature of the electrode—the electrode must have a very large surface area in order to be able to take advantage of the potential for high energy density. Further, a proper PC material and electrolyte or ionic liquid is required as well. Still further, a high energy density pseudocapacitor must be constructed of lightweight, low cost, non-toxic materials in order to be commercially viable. So far, all known methods for creating a PC electrode involve coating of a PC material onto an inactive substrate, which only adds mass without contributing to energy storage and reduces the stored energy density.
U.S. Pat. No. 7,084,002 to Kim et al. describes a similar templating method employing sputtering of a metal onto the anodized aluminum oxide template, a method that will not work for the ultrahigh aspect ratios of the nanoscale pores required for the electrode to work properly and to its highest energy storage potential due to the directional nature of the deposition process and shadowing effect of a deposited material upon any structure underneath. In addition, U.S. Pat. No. 7,084,002 requires electrochemical deposition of appropriate metal oxides, which cannot not occur on insulating aluminum oxide templates. Similarly, U.S. Pat. No. 7,713,660 to Kim et al. describes wet chemical processes that cannot achieve the wall thickness control or arrayed attachment to a conductive substrate. Further, capillary and surface tension effects limit the tube diameters to dimensions greater than hundreds of nanometers under this method.